Digital data storage apparatus is commonly used for the storage of large quantities of data, usually in the form of multiple-bit words. Data may be written into the store at any location by addressing that location and applying the data to be stored there. Similarly the data at any location may be read out by addressing that location and activating the read function. Many types of store operate in this simple manner. It is common practice that, before a section of storage is to be used for some new purpose, it is cleared in order to remove any residual data left from an earlier operation, and thus reduce the possibility of a malfunction. Sometimes this clearing operation is carried out immediately after the completion of an operation using the section of storage, particularly when confidentially of data is important, in order to prevent accidental disclosure.
In order to identify whether the contents of the store have any meaning or not, it is not uncommon practice to add to each multiple-bit word an extra bit called a "validity" bit. This bit is arranged to be "set" when data is written into a store location, and is "cleared" before a section of store is brought into use. Hence one action when reading from a store location is to examine the validity bit. If the bit is "clear", then the output from the store at that location is invalid. This means that data in the store is meaningless, and may be forced to zero. What follows is that, instead of actually clearing every bit position in an area of store to be cleared, it is only necessary to clear the validity bits, since no output can then be obtained from that area. It is usually necessary to address every validity bit separately, since each has a different address, in order to clear a general area of store. If a large area of store is to be cleared this can take a relatively long time.
It is an object of the invention to provide digital data storage apparatus in which a consecutive set of validity bits may be set or cleared in a very short time.